Microlithography is used for the production of micro structured components such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination system and a projection objective. In that case the image of a mask (=reticle) illuminated via the illumination system is projected via the projection objective on to a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure on to the light-sensitive coating on the substrate.
Mirrors are used as optical components for the imaging process in projection objectives designed for the EUV range, that is to say at wavelengths of for example about 13 nm or about 7 nm, due to the lack of availability of suitable translucent refractive materials. A problem which arises in practice is that the EUV mirrors experience a rise in temperature and linked thereto thermal expansion or deformation, as a consequence of absorption of the radiation emitted by the EUV light source but also because of cleaning in a hydrogen atmosphere, and such expansion or deformation in turn can result in worsening of the imaging properties of the optical system. To be able to assess those effects and possibly compensate for them there is a need to determine the extent of the mirror heating as accurately as possible.